Applicants claim, under 35 U.S.C. xc2xa7 119, the benefit of priority of the filing date of Jan. 17, 2002 of a German Patent Application, Serial Number 102 01 496.5, filed on the aforementioned date, the entire contents of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a scale which is suitable for an absolute position determination, including a track which extends in at least one measuring direction and in which graduation areas of identical width and different optical properties are alternatingly arranged, and a first and second logical signal is unequivocally assigned to the preset sequence of two successive graduation areas. The present invention furthermore relates to a position measuring system for an absolute position determination including a scale, as well as a scanning unit, which is movable in the measuring direction for determining the absolute position of the scanning unit in relation to the scale by scanning the scale.
2. Discussion of Related Art
Incremental position measuring systems are known for determining the position of two objects which can be moved in relation to each other and which detect the relative movements of the objects in relation to each other in defined measuring steps, or increments. As a rule, the incremental position measuring systems employed for this purpose include a scale with a track in an incremental measuring graduation, which is connected with one of the two objects, as well as a scanning unit, which is connected with the other of the two objects. Displacement-dependent periodic incremental signals are generated in a known manner by optical, magnetic, inductive or capacitive scanning.
Besides this, so-called absolute position measuring systems are known, which include a track with a sequential code of several bit words on the part of the scale, which for example are designed as a pseudo random code, and from the scanning of which the absolute position along the respective measuring path can be determined. Here, the respective sequential code in the measuring direction includes an appropriate selected sequence of logical signals, or bits, which assume the values ZERO (0) and ONE (1), for example. For increasing the detection dependability in connection with such systems it is now known to derive each individual logical signal, or bit of a bit word, from a predetermined sequence of two graduation areas of different optical properties. For example, the logical signal ZERO (0) corresponds to the sequence of a transparent and a non-transparent graduation area, but the logical signal ONE (1) corresponds to the sequence of a non-transparent and a transparent graduation area in the track. Such a coding is called Manchester coding, in connection with this, reference is made to FIG. 1 in the publication xe2x80x9cAbsolute Position Measurement Using Optical Detection of Coded Patternsxe2x80x9d by J. T. M. Stevenson and J. R. Jordan, in the J. Phys. E. Sci. Instrum. 21 (1988), pp. 1140 to 1145.
Two typical problem areas result when using such Manchester codings in absolute position measuring systems.
Initially, it must be assured in principle that the graduation areas are correctly read out, or that the detector elements used for this are correctly selected in order to assign the correct logical signal, or the correct bit value ZERO (0) or ONE (1), to a defined sequence of two graduation areas. The above mentioned publication does not provide any further suggestions in this regard.
Furthermore, the absolute position information generated by the Manchester coding is not sufficiently fine with respect to resolution. This means that as a rule provisions are made to combine the absolute position information with position information from a finer resolving incremental position measurement. For creating a higher resolving position information from a roughly resolving absolute code information, which can be combined with the incremental position measurement, initially the transitions between different graduation areas, and therefore a rough position within the respective code word, are determined in the just scanned code word by an edge interpolation, with which the incremental information is then combined. Such a process is also proposed in the above mentioned publication. It has been shown to be problematic in this case that the resultant accuracy of such an edge interpolation is negatively affected by diffraction effects, as well as the possibly existing divergence of the light source, so that errors in the position determination can result. This is the case in particular when higher resolving position measuring systems are employed with narrow widths of the graduation areas.
It is therefore an object of the present invention to disclose a scale for a position measuring device which is suitable for an absolute position determination, and by which a dependably readable absolute position signal of high resolution can be obtained from a single track.
This object is attained by a scale which is suitable for an absolute position determination, the scale includes a track which extends in at least one measuring direction and in which graduation areas of identical width and different optical properties are alternatingly arranged. At least first, second and third graduation areas with different optical properties that are arranged in the track, wherein a first logical signal is unequivocally assigned to a first combination of two successive different graduation areas, and a second logical signal is unequivocally assigned to a second combination of two successive different graduation areas, and wherein the first and second combinations differ from each other.
It is a further object of the present invention to disclose a position measuring system for an absolute position determination, by which a dependably readable absolute position signal of high resolution can be generated from scanning a single track.
This object is attained by a position measuring system for absolute position determination. The position measuring system includes a scale that includes a track which extends in at least one measuring direction and in which graduation areas of identical width and different optical properties are alternatingly arranged. The scale further includes at least first, second and third graduation areas with different optical properties that are arranged in the track, wherein a first logical signal is unequivocally assigned to a first combination of two successive different graduation areas, and a second logical signal is unequivocally assigned to a second combination of two successive different graduation areas, and wherein the first and second combinations differ from each other. A scanning unit, which is movable in relation to the scale in a measuring direction and determines an absolute position of the scanning unit in relation to the scale by scanning the scale.
It is now provided in accordance with the present invention to employ on the part of the scale at least three graduation areas with different optical properties in a track. In the course of this, a first logical signal is unequivocally assigned to a first combination of two successive different graduation areas, and a second logical signal is unequivocally assigned to a second combination of two successive different graduation areas. The first and second combinations differ from each other.
Thus an unequivocal coding of the just read out bit word position is assured by these steps without further information being required. Added to this is that it is also possible to assure the detection of possibly erroneously read out signals.
Because of the periodic arrangement of one of the graduation areas on the scale it is possible to assure by appropriate measures in connection with scanning that, besides the absolute position information, moreover at least one rough incremental signal can be generated and can be employed for forming a highly resolved absolute position value. In this case no edge interpolation, such as explained above, is required, i.e. it is now possible on the basis of considerations in accordance with the present invention to produce highly resolving position measuring systems.
Basically every one of the three different graduation areas can be periodically arranged on the scale, i.e. the first, as well as the second or third graduation areas.
Regarding the embodiment of the third graduation areas there are a number of options if the first and second graduation areas have complementary optical properties. For example, in a possible variation of the third graduation areas it is possible to choose an optical property which lies between the optical properties of the complementarily embodied first and second graduation areas. In a further variation the third graduation areas have a periodic structure from which an additional fine incremental signal can be derived, which is used for once more increasing the resolution of the incremental position determination.
Because of the steps in accordance with the present invention, in a possible embodiment only a single detector arrangement with several detector elements, by which all scanning signals can be generated, is required on the scanning side for generating the different scanning signals.
The present invention can of course be used in transmitted light systems, as well as in incident light systems. In the same way linear, as well as rotatory position measuring systems can be designed. The considerations on the basis of the present invention can furthermore be transferred to systems which include more than three different graduation areas on the scale side.